Cell sorting is of tremendous importance not only for basic cell biology but also for clinical medicine, cancer research, reproductive medicine or transplantation immunology. Modern cell sorting schemes operate in several different ways. For example, cells may be sorted in continuous flow or encapsulated in small liquid droplets prior to sorting. In the latter case, the problem of sorting applies to the droplets and not to the cells. Droplets can be sorted, for instance, in air or in another immiscible continuous liquid. Traditional fluorescence activated cell sorters (“FACS”) encapsulate cells in droplets, which are then labeled with an electric charge and subsequently separated in an electric field. These sorters reach very high sorting rates, but have several disadvantages including high costs and large dead volume, which make it nearly impossible to separate cells from small sample volumes. Moreover, elaborate cleaning and maintenance procedures are necessary to prevent cross-contamination of different samples, making handling more difficult.
These drawbacks can be avoided using low cost disposable microfluidic devices which operate at small sample volumes. In such devices, highly monodisperse aqueous droplets enclosing the cells can be produced at very high rates in an immiscible continuous oil phase instead of air. Such emulsions can even be prepared having higher hierarchies, e.g., in so-called “multiple emulsions,” containing droplets in droplets. In single emulsions, the objects to be sorted (e.g., cells) can be distinguished from the bulk solution, for example, because of their inherent contrast in material properties of the aqueous and oil phases. This contrast can be exploited for sorting in some cases. Most commonly used is the polarizability contrast in dielectrophoretic sorters. Other sorters can be found in U.S. patent application Ser. No. 11/360,845, filed Feb. 23, 2006, entitled “Electronic Control of Fluidic Species,” by Link, et al., published as U.S. Patent Application Publication No. 2007/0003442 on Jan. 4, 2007, incorporated herein by reference.
However, many droplet-enhanced sorters come with an additional processing step of loading cells into the droplets. In some cases, enclosing the cells in droplets may not be desirable; for example, if the cells are to be cultured after sorting, they must be first removed from the emulsion.
In contrast to droplet sorting, direct cell-sorting schemes operating in the continuous phase have to deal with low contrast of material properties of cells and the bulk solution containing the cells, as both typically appear as aqueous liquids. To overcome this limitation, responsive beads are often biochemically attached to the cells to enhance the separation efficiency. For example, in magnetic activated cell sorting (MACS), a magnetic bead is selectively adhered to a target cell prior to sorting the cell using a magnetic field. Also, attachment of polarizable beads has been used to subsequently separate the target and waste cells in an electric field gradient. Optical force switching has been used for sorting as well but suffers from relatively slow sorting rates.
There are also a few techniques that utilize hydrodynamic flow to sort cells such as syringe enhanced pumping or electrokinetic mobilization. Typically, they all suffer from slow response times and consequently low sorting rates or low cell viability under high electric fields.
Accordingly, improvements in cell sorting devices and methods are needed.